In a sine wave drive system (180° energization system) whose application examples are increased as drive systems of synchronous motors (permanent magnet synchronous motors), a control of detecting a rotor position (a rotational position of the rotor) in a sensorless manner and performing the suitable energization to a stator coil is executed. As a motor control device having a function of detecting the rotor position in a sensorless manner, a motor control device disclosed in Patent Document 1 has been proposed. The motor control device of Patent Document 1 determines a voltage phase of an applied voltage by a phase current detecting means for detecting a phase current of a synchronous motor, a current phase calculating means for calculating a current phase based on the detected phase current, and a voltage phase setting means for adding a predetermined phase difference to the calculated current phase to convert it to a voltage phase. Moreover, the applied voltage to the synchronous motor is set based on the voltage phase, and a command voltage determined according to an operation command.
According to the motor control device of Patent Document 1, the phase instruction value of the applied voltage to be applied to the synchronous motor is set by adding a predetermined phase difference to the detected current phase. The phase difference to be added is calculated based on the detected current peak value, an angular velocity of an induced voltage, and a target current phase (lead angle from a q-axis), or is obtained by referring to a data table from the current peak value and the angular velocity of the induced voltage.